An inverter in a photovoltaic power generation system serves to convert direct current (DC) power generated in a photovoltaic panel to alternating current (AC) power. The inverter starts an operation when direct current input power is equal to or larger than a predetermined level Win-start necessary for a normal operation, and stops for protection of a device when the direct current input power is equal to or larger than maximum input power Win-max. The inverter stops the operation at minimum input power Win-min. Here, values of the Win-min and the Win-start may be the same as or different from each other according to the inverter.
Efficiency of the inverter is defined as a ratio of input power to output power, and does not always have a uniform value through the whole operation range, and is changed according to the output as illustrated in FIG. 1. The efficiency of the inverter is different according to a structure and a control method, but it is generally known that the efficiency of the inverter is highest in a range from 30% to 80%.
The inverter of the photovoltaic power generation system may be divided into a module-integrated converter (MIC), a string inverter, a multi-string inverter, a central inverter, and a multi-central inverter according to a type of a combination of a photovoltaic panel and an array.
The MIC, which is a type of attaching an small inverter for each panel, has an advantage in that installing is easy because a separate DC line wiring is not necessary, and it is possible to harvest maximum energy even when daylight conditions between panels are different due to shadow or a difference in installation conditions, but has a disadvantage in that a cost burden is large when large capacity is implemented and efficiency is slightly lower than the large capacity inverter. The MIC starts to be supplied based on an advantage, such as flexibility and expandability of a panel arrangement in a small system, such as building integrated photovoltaics (BIPV), including a home system.
The string inverter method, which is a method using a DC/AC inverter for each serial panel group, may control maximum power point tracking (MPPT) for each string, and may relatively effectively harvest energy for a partial shadow. However, when the string inverter method is applied to a large capacity power plant, maintenance and repair costs are increased because the number of inverters is excessive, and the string inverter method is slightly inappropriate in an aspect of protection of a system, such as prevention of an isolated operation, because it is impossible to perform a central control of the inverter, such that the string inverter method is appropriate to a photovoltaic power generation system with a middle capacity.
The multi-string inverter method, which is a method using a DC/DC converter for each serial panel group, is a form including the advantages of the string method and the central method, but has a disadvantage in that efficiency of the system is slightly low due to the dual stage power conversion.
The central inverter method has a disadvantage in that energy harvest is slightly low due to a serial-parallel combination of all of the panels, but has an advantage in that efficiency of the converter is excellent, and costs compared to output capacity are low, so that the central inverter method is mainly used as a large capacity inverter method for industry. The central inverter method uses a single inverter, so that the central method has an advantage in that it is advantageous to protect the system and maintenance and repair costs are small, but has a disadvantage in that the whole system fails to be operated when the inverter fails. Recently, in order to supplement the aforementioned disadvantages, a multi-central inverter method of implementing one large capacity inverter system by connecting large capacity central inverters in parallel has been widely developed.
The inverter adopting the multi-central inverter method has a structure in which inverters adopting the central method are connected in parallel, and includes a plurality of inverters, not one inverter, when the power generation system is formed. The inverter is allowed to be operated under an optimum condition by operating only a specific inverter by collecting power generated in photovoltaic panels under a condition where photovoltaic energy is low, such as sunrise, sunset, and cloudy weather, and operating all of the plurality of inverters when photovoltaic energy is large, thereby improving efficiency of photovoltaic power generation equipment. The inverter adopting the multi-central method has an advantage in that the a use life of the inverter is extended by sequentially operating the inverters so that operating times of the inverters are equally maintained, and when one inverter fails, or is maintained and repaired, another inverter may be operated at a high energy level, so that it is possible to reduce energy loss, thereby starting to be supplied to a large-scale photovoltaic power generation system.
However, the multi-central inverter method needs to control the plurality of inverters and the photovoltaic panels, so that system building costs are increased, and a complex control function, including communication between the inverters or between the inverter and a central control device, is demanded, so that the multi-central inverter method is disadvantageously inappropriate to a small photovoltaic power generation system.